1. Field of the Invention
The present invention relates to a method of and an apparatus for preventing accidents caused by blockage of a rotatable tool when working with a hand-held tool including the rotatable tool, a drive motor for driving the rotatable tool, and means for interrupting transmission of a drive torque from the drive motor to the rotatable tool dependent on an operational condition of the hand-held tool determined with a displacement measurement device.
2. Description of the Prior Art
Accidents, which are caused by rotatable tools, in particular injuries in the region of the wrist or the arm, or fall of ladders, of a scaffold and so on, often results from a sudden blockage of the rotatable tool and by a resulting rapid increase of the reaction torque of hand-held tool equipped with the rotatable tool, in particular, when a high-power hand-held tool, such as a drill hammer is used. The danger of such accidents was recognized since long ago. Different solutions for solving the problems associated with the blockage of a rotatable tool have been disclosed, e.g., in European Publication EP 150 669 A2 and in International Publication WO 88/06508 A3. Accordingly to these solutions, by using a torsional sensor, in particular, an acceleration sensor arranged in or on a hand-held tool housing, which senses acceleration or outer pivotal movement or displacement of a hand-held tool and generates an appropriate output signal, a drive train between a drive motor and the rotatable tool, in particular, between the drive train and the rotary spingle is broken, when in accordance with a predetermined criterium, e.g., an acceleration threshold, a clutch is actuated when the output signal of the sensor exceeds the predetermined criterium. The drawbacks of these solutions, which are proposed in the above-mentioned prior art, consist in an erroneous actuation of the clutch even at a normal operation of a hand-held tool, e.g., during the use of a hammer drill for drilling in a concrete mass having an unhomogeneous composition. This is associated basically with an immediate evaluation of the sensor output signal without a preliminary assessment of the signal, i.e., evaluation of the output signal using inevitably comparatively low threshold values, without an individual assessment of a respective signal.
A significant improvement was achieved by using an evaluation method with a preliminary assessment for signals outputted by an acceleration sensor, which is described in German Patent No. 4,344,817. The improved method consists in calculating in advance, based on a rotational acceleration variable which is generated by an acceleration sensor based on a reaction torque caused by blockage or partial blockage of the rotatable tool, and on a predetermined time constant, an expected twist angle of the hand-held tool, and in actuating the safety clutch when the calculated or expected twist angle exceeds a predetermined maximum allowable twist angle. Thereby, a future blockage of the hand-held tool is evaluated immediately after an occurrence of a blockage, and counter-measures are undertaken when the hand-held tool is subjected to a rotary pulse capable of causing an accident.
However, the experiments have shown that the method described in German patent No. 4,344,817, though advantageous, has two serious drawbacks, namely:
(i) the rotational axis of the tool often, at the critical point of the blockage, does not coincide with the tool axis; and PA1 (ii) the acceleration caused by gravity influences the measurement signal of the acceleration sensor dependent on an immediate position of the tool.
Accordingly, an object of the present invention is to improve a hand-held tool of the type described above in such a way that a measurement signal, which is generated by an acceleration sensor or sensors in response to a reaction pulse or a reaction torque upon blockage of the working tool, provides an unambiguous information whether a dangerous blockage has occurred when the rotational axis of the tool becomes twisted.
Another object of the present invention is to improve the hand-held tool of the type described above in such a way that the influence of the gravity acceleration on the measurement signal is eliminated.